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Add *ready* plugin (#2616)

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Add a ready plugin that allows plugin to signal when they are ready.
Once a plugin is ready it is not queried again.

This uses same mechanism as the health plugin: each plugin needs to
implement an interface.

Implement readines for the *erratic* plugin to aid in testing.

Add README.md and tests moduled after the health plugin; which will be
relegated to just providing process health. In similar vein to health
this is a process wide setting.

With this Corefile:
~~~
. {
    erratic
    whoami
    ready
}

bla {
    erratic
    whoami
}
~~~

ready will lead to:

~~~ sh
% curl localhost:8181/ready
% dig @localhost -p 1053 mx example.org
% curl localhost:8181/ready
OK%
~~~

Meanwhile CoreDNS logs:

~~~
.:1053
bla.:1053
2019-02-26T20:59:07.137Z [INFO] CoreDNS-1.3.1
2019-02-26T20:59:07.137Z [INFO] linux/amd64, go1.11.4,
CoreDNS-1.3.1
linux/amd64, go1.11.4,
2019-02-26T20:59:11.415Z [INFO] plugin/ready: Still waiting on: "erratic"
2019-02-26T20:59:13.510Z [INFO] plugin/ready: Still waiting on: "erratic"
~~~

*ready* can be used in multiple server blocks and will do the right
thing; query all those plugins from all server blocks for readiness.
This does a similar thing to the prometheus plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>
This commit is contained in:
Miek Gieben
2019-03-07 20:35:16 +00:00
committed by GitHub
parent 2b7e84a076
commit db0b16b615
13 changed files with 409 additions and 2 deletions
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# ready
## Name
*ready* - enables a readiness check HTTP endpoint.
## Description
By enabling *ready* an HTTP endpoint on port 8181 will return 200 OK, when all plugins that are able
to signal readiness have done so. If some are not ready yet the endpoint will return a 503 with the
body containing the list of plugins that are not ready. Once a plugin has signaled it is ready it
will not be queried again.
Each Server Block that enables the *ready* plugin will have the plugins *in that server block*
report readiness into the /ready endpoint that runs on the same port.
## Syntax
~~~
ready [ADDRESS]
~~~
*ready* optionally takes an address; the default is `:8181`. The path is fixed to `/ready`. The
readiness endpoint returns a 200 response code and the word "OK" when this server is ready. It
returns a 503 otherwise.
## Plugins
Any plugin wanting to signal readiness will need to implement the `ready.Readiness` interface by
implementing a method `Ready() bool` that returns true when the plugin is ready and false otherwise.
## Examples
Let *ready* report readiness for both the `.` and `example.org` servers (assuming the *whois*
plugin also exports readiness):
~~~ txt
. {
ready
erratic
}
example.org {
ready
whoami
}
~~~
Run *ready* on a different port.
~~~ txt
. {
ready localhost:8091
}
~~~